Gauss

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The last name of Carl Friedrick Gauss, one of the greatest mathematicians in history. His work in calculus was (and is) invaluable to the study of electromagnetism, so he got a unit of magnetic field strength named after him.

It is universally agreed that "Gauss" is a cool name, and so you can bet dollars to donuts that any sci-fi gizmo with any sort of vaguely electromagnetic theme will have the word "gauss" in it.

By the way, if an electromagnetic weapon doesn't have "gauss" in its name, it's a fair bet that it has the word "tesla" in it, after another cool-sounding unit of magnetic field strength, itself named for Nikola Tesla, a famed inventor and scientist who did cool stuff with electromagnetism.

"Realistic" gauss weapons come in two flavours: rail-gun and coil-guns. Rail-guns work by having a really long pair (or pairs) of conductors, and passing opposing currents through them and the ammunition. It works like an electric motor that is unwound into a straight line. Cheap to manufacture, awesome velocities, but it needs to be really long and causes a lot of stress on the materials. Coil-guns work by surrounding the barrel with a sequence of coil magnets that are switched on and off to pull&push the ammunition. Since the ammunition doesn't physically touch the mechanisms, the gun lasts longer and there is better control over the speed, but the control mechanisms are very complicated. Of course, once you get into soft sci-fi, all bets are off; expect to see guns that shoot lightning.

This has nothing to do with the Peasant Railgun.

Real-Life

The US Navy has successfully tested 10-megajoule 32-megajoule railguns. The friction from the ammunition moving at these velocities turns the air inside the barrel into plasma instantly. They were planning to build a 64-megajoule testfire, but they lost funding; might have to do with how they had to build an entirely new railgun after every third shot.

If you see a carpenter using an "electric nailgun," it's actually a coilgun they're using.

Rifts

"Gauss cannon" is just a synonym for "machinegun that does MDC damage." The artwork for railguns always shows them as ejecting cartridges and being round, with no rails in sight ... it's not like anything else in Rifts even tries to be rational, so just roll with it.

Traveller

Gauss weapons are slugthrowers that use 3-5.5mm (.11 to .21 calibre) slugs or flechette needles. They're the preferred rifle and sidearm for TL12 armies (Tech Level 10 in GURPS Traveller); at this tech level, the preferred heavy weapons will be laser or plasma. Until these non-chemical sidearms are in use, ship boarding parties in space prefer using sabres and melee for combat. The exception is zero-G environments, where recoil on any slugthrowers (gauss or not) are as much a hazard for the wielder as the target.

Warhammer 40,000

Tau

The Tau use railguns as the primary heavy weapons on tanks and walkers. In these cases they are very powerful and penetrating (as in real life) as the acceleration is limited only by the amount of power that can be input and how much the weapon can handle with out exploding or melting (in the case of these two, it is quite a lot; rumor has it that in their next Codex, Tau railguns will wound all units in a line).

• Railgun: The Railgun is the iconic Tau heavy weapon, mounted on Battlesuits and Hammerhead Gunships to cause massive damage to enemy armor (72", S10, AP1). A Broadside Battlesuit Team can carry up to 3 twin-linked railguns in one Heavy Support slot, while the Hammerhead can only carry one, but the Hammerhead has enough ammunition capacity to also carry a S6 AP4 Large Blast submunition round for dealing with infantry blobs.
• Heavy Railgun: Because the regular railgun wasn't enough for putting down the really big targets (like Titans), the Earth Caste developed the Heavy Railgun (110", SD, AP1) for their super-heavy vehicles. The Manta mounts two of these, while the Tiger Shark AX-1-0 mounts a twin-linked pair. Like the Hammerhead railgun, the heavy railgun can fire a pie-plate for destroying massed infantry formations, but the heavy railgun's submunitions are S7 AP3 and the blast is 10" across.
• Rail Rifle: Emboldened by the success of vehicle-mounted railguns, the Earth Caste built a Tau-portable railgun weapon, called the "rail rifle." It debuted in the Fire Warrior video game as a prototype weapon; it soon got rules in Chapter Approved, and by the next Tau Empire Codex, it was a regular part of the Fire Caste's armory, used by Pathfinders and drones as a sniper weapon. Incidentally, between being Chapter Approved and included in the 4th Edition Codex, the Rail Rifle was subject to "Gets Hot!" like a Plasma Gun, but the Earth Caste eventually managed to improve the safety margins on it such that blowing up in the user's face is no longer a concern.
• Heavy Rail Rifle: A recent development wielded by next-generation Broadside battlesuits, it is actually less powerful than the Tau's primary railguns, though only moderately so (S8 versus S10). However, its reduced bulk allows it to be mounted in such a way as to more easily track fast moving targets, and thus it fulfills an anti-air role, where its slightly reduced strength is not a big liability as it is still plenty powerful enough to punch through most fliers with ease, and its double-barreled (twin-linked) setup increases its odds of hitting a target.

Necrons

Most Necron weapons have the word "gauss" in them, but they all shoot some kind of molecule-stripping super-lightning rather than a magnetically-accelerated projectile. In the Third Edition Codex, Gauss weapons automatically wounded infantry and automatically caused glancing hits to vehicles on a to-hit roll of 6. This was devastatingly powerful at the time, as it meant that Necrons could chew through vehicles like no tomorrow, but was rather reduced in effectiveness in Fifth Edition, as most vehicles could no longer be destroyed through glancing hits alone, though with the introduction of Hull Points in Sixth Edition, Gauss weapons are once again a lot more dangerous to vehicles (three glancing hits -- which a full squad of 20 Necron Warriors can easily supply -- is enough to wreck most vehicles). That said, Gauss weapons were reduced in effectiveness by the Fifth Edition Codex, which removed the auto-wound property.

The anti-infantry effectiveness was moved to the Necrons' new "tesla" weapons. Those things are horde-killing assault guns that fire off high-strength bolts of lightning that ground themselves on the enemy. Their gimmick is that each to-hit roll of six grants two additional automatic hits on the same unit. Like Gauss weapons only kinda insta-gib vehicles, these weapons only sometimes insta-gib infantry.

• Gauss flayer: The trusty Gauss flayer is the standard weapon (in fact, the only weapon) wielded by Necron Warriors. Its stats are equivalent to the bolter, with the additional "Gauss" rule mentioned above. Ghost Arks and Doomsday Arks mount an array of five of these guns on each side.
• Gauss blaster: The Gauss blaster is the standard weapon of Necron Immortals; it is basically a Gauss flayer with slightly improved strength and armor-piercing capability. A unit of Tomb Blades can also choose to take twin-linked pairs of them to specialize in vehicle-hunting.
• Gauss cannon: The iconic weapon of Necron Destroyers is their shoulder-mounted gauss cannon. Though it's only as strong and long-ranged as a gauss blaster, it has more shots and will tear through all but the toughest armor suits.
• Heavy gauss cannon: Necron Heavy Destroyers get heavy gauss cannons, and Triarch Stalkers can choose a twin-linked set of them as a primary weapon. They only get one shot, but it's as powerful as a lascannon.
• Gauss flux arc: The Gauss flux arc is basically like the Gauss flayer, but firing more shots at a time. Monoliths mount one at each corner, and they are capable of choosing their targets independently.
• Tesla carbine: A unit of Immortals or Tomb Blades can choose to use tesla carbines instead of gauss blasters, sacrificing the Gauss rule and all armor-piercing ability for the chance to land extra hits.
• Tesla cannon: A tesla cannon is like a tesla carbine with another point of strength and twice as many shorts. Annihilation Barges and Catacomb Command Barges mount one slung underneath the operators' pedals.
• Tesla destructor: Again, another point of strength and twice as many shots compared to the tesla cannon. The Annihilation Barge, Night Scythe, and Doom Scythe all mount a twin-linked set of them for vaporizing blobs. The tesla destructor's bolts are so powerful that they can arc extremely far from their initial targets; any unit within 6" of a unit attacked by a tesla destructor gets hit by d6 Strength 5 AP- hits on a roll of six.

Eldar

Eldar shuriken weapons and Dark Eldar Shard weapons both use coilgun technology to throw their ammo. Which is strange, because neither psychoplastic nor shardcrystals are ferromagnetic, and so cannot be accelerated with magnetic fields.

Imperium

Strangely enough, heavy bolters use coilgun tech to further accelerate its bolts, so they can use much lighter ammo with less gunpowder. Also, the NOVA CANNON is sometimes described as fuckhuege railgun, though this is only one of a half dozen completely different explanations of how the NOVA CANNON actually works.